Signals are transmitted from a transmitter to a receiver through various propagation paths in a wireless communication system. The propagation path changes over time according to a factor such as a multipath.
A channel environment can be classified into frequency flat fading and frequency selective fading occurring due to dispersion in a time domain when delay spread is achieved in the time domain. A Doppler frequency depending on a movement speed of a user equipment causes dispersion in a frequency domain, which leads to generation of fast fading and slow fading which are distinguished by a channel change in the time domain. The dispersion in the time domain is independent from the dispersion in the frequency domain.
When data passes through a frequency flat fading channel in a conventional multiple input multiple output (MIMO) system, dispersion constituted by channel gain values is not high but is similar throughout the entire frequency band. If a channel gain is high in the entire frequency band, an overall link throughput of the system is good. If the channel gain is low in the entire frequency band, the overall link throughput of the system may deteriorate. In addition, the MIMO system causes a significant computational complexity in the receiver.
A diversity technique for repeatedly transmitting the same data is developed to ensure communication reliability according to various propagation paths. When the same data is transmitted through different independent paths, even if a signal of a specific path is received with low signal strength, signals of the remaining paths may have high signal strength values. Examples of the diversity include frequency diversity that transmits data with different frequencies, time diversity that transmits data at different time points, and spatial diversity that uses a plurality of transmit (Tx) antennas.
Cyclic delay diversity improves a link throughput in such a manner that a frequency diversity gain is obtained by artificially increasing selectivity of a channel in the frequency domain. Data transmitted to the receiver through multiple antennas experiences different time delays. Therefore, according to a time delay, the receiver can estimate and detect data transmitted through a corresponding Tx antenna, thereby reducing the computational complexity. However, the cyclic delay diversity is not adaptive to a channel environment in the time domain. That is, even if the overall throughput of the system is affected by both a channel change in the time domain and a channel change in the frequency domain, there is a demerit in that the cyclic delay diversity is not adaptive to the channel change in the time domain.
An apparatus for regulating time-frequency selectivity is disclosed by the PCT international application No. PCT/KR2007/003625 filed by the applicant of the invention. The apparatus provides time diversity, frequency diversity, and multi-user diversity for channels, each of which independently operates in the time domain and the frequency domain.
However, a method of adaptively regulating channel selectivity when the channel selectivity is high in the time domain or the frequency domain is not disclosed.